The present invention relates to a method for relieving residual stresses in the inner surface of a weld by heating the outer surface of a butt welded joint between pipe sections by a welding machine under predetermined conditions and by utilizing the temperature difference between the outer and inner surfaces of the pipe sections.
When pipe sections are butt welded, high tensile residual stresses are left in the weld and the heat affected zone. If the residual stresses are not relieved, stress corrosion cracking results so that in the cases of pipes and cylindrical or tubular vessels used in nuclear reactors, chemical plants or the like, there exists a fear that a serious accident will happen. Therefore, the residual stresses must be relieved. Generally, the welded joints are subjected to such heat treatments that the tensile residual stresses in the inner surfaces of the weld and the heat affected zone are relieved or converted into the compressive residual stresses.
There are various methods for relieving the residual stresses by utilizing the temperature difference between the inner and outer surfaces of a welded pipe. They are, for instance, a high frequency heating process and the Linde process (for heating the outer surfaces of a welded pipe by flames). The high frequency heating process requires a large-sized device, and a high frequency induction coil must be fabricated for each pipe so that the high frequency heating process is expensive. Furthermore the high frequency heating process requires many days. According to the Linde process, it is technically difficult to produce stably residual stresses in the inner surface of a welded pipe in a stable manner.
Neither the high frequency heating process nor the Linde process can provide satisfactory conditions for relieving the residual stresses. Both processes are carried out based on experience so that satisfactory stress relief cannot be accomplished.
In view of the above, the present invention provides a method for relieving the residual stresses at a welded joint in a simple yet very positive manner. According to the present invention, a welding machine such as a TIG welding machine, a MIG welding machine, a plasma arc welding machine or the like is used so that weld heat input is so controlled that a local temperature gradient for relieving the residual stresses can be developed.